Differential enhancement of periodically variable images

ABSTRACT

Image signals are converted to video signals which are differentially detected, integrated, subtracted and displayed to display integrated differential features.

United States Patent Mistretta et al.

July 8, 1975 DIFFERENTIAL ENHANCEMENT OF PERIODICALLY VARIABLE IMAGES Inventors: Charles A. Mistretta; Michael G.

Ort, both of Madison, Wis.

Assignee: Wisconsin Alumni Research Foundation, Madison, Wis.

Filed: June 14, 1973 Appl. No.: 369,824

References Cited UNITED STATES PATENTS 10/1966 McMaster 178/DIG.

6/1971 Siedband 178/DIG. 33

2/1972 Stults 178/DIG. 33 7/1972 Corrigan 178/DIG. 5

Primary Examiner-Howard W. Britton Attorney, Agent, or FirmBurmeister, Palmatier &

l78/DIG. 33, DIG. 38

ABSTRACT Image signals are converted to video signals which are differentially detected, integrated, subtracted and displayed to display integrated differential features.

14 Claims, 7 Drawing Figures 44-7 7 V1 DE VIDEO 54 mi, 8%? 55553;; 42 4am) 480. 40/ TV L 7 CAMERA 12 INTEGRATING 123 U W 9 5UBTRACTIDNS 12 STORAGE/DEVICE X-RAY l 2s 24 PULsER -D I a. TV MONITOR 34 R541, as;

POWER J SUPPLY 6v. 20v. 30v. 0v. 340v. 704. v2 r ELEC- ELEC, ELEC. ELEC. TRON/C 5M]I SW SW VERT SYNC 554' 787 puisf CONTROL PULSE 7,2 ELEC. GENERATOR GENERATOR 501 SW. I Tv SWEEP VERTICAL SWEEP 4 GENERATOR ,.HORIZONTAL SWEEP QATFMTEDJUL 8 I915 I 3.894.181

SHEET I 106 I08 I a 1 22 112 Q POSITIVE CHARGE I DEPOSITED m I l 255 .30 ab 90 g TARGET ISLAND VOLTAGE 8 L6); 1 EGATIVE CHARGE DEPO S I TED P'A'TH- TEDJUL 8 ms SHEET RAY ENERGY (Kev) QUASl-MONOENERGETIC X- RAY SPECTRA 0 AL m 0 QEEQUEOQ 5:525? 51 k vp SPECTRUM 40EN M 5) (Rev) M 98mm DIFFERENTIAL ENHANCEMENT OF PERIODICALLY VARIABLE IMAGES This invention relates to the enhancement of differential features which may be present in repetitive timedependent images so that the contrast of such differential features will be increased to a point such that the differential features are clearly visible, even though initially the differential features may have such low contrast that they are scarcely visible.

The present invention will find many applications to situations where certain features of images are periodically variable. For example, the present invention is particularly well adapted for enhancing the contrast and visibility of differential features in X-ray images. Such differential features may be produced by periodically changing the conditions under which the X-ray images are produced. For example, two or more different X-ray filters may be moved repeatedly into and out of the path of the X-rays so as to vary the energy of the X-rays.

The X-ray conditions can also be varied by repetitively pulsing the X-ray source to two or more different voltages so as to change the energy of the X-rays. It is also possible to switch repetitively between two or more X-ray sources having different energies. The differential features in the X-ray images can also be produced by time-dependent variations in the subject being X-rayed. For example, such time-dependent variations may be caused by the heart beat or the respiration of the subject.

The general object of the present invention is to enhance the contrast and visibility of the differential features of periodically variable images, particularly X-ray images.

In accordance with the present invention, a television system is employed to convert the periodically variable image into video signals. In this way, the differential features of the periodically variable image are reproduced in the form of differences between the sequential sets of video signals representing frames of the television images. The sequential video signals are supplied to a video difference detector which develops differential video signals. Such signals are supplied to an integrating and subtracting storage device which integrates the differential features of the video signals, while subtractively combining the non-differential features. The integrated video signals are then supplied to a TV monitor which displays the integrated differential features of the video images.

In the illustrative embodiment, first and second X-ray images having differential features are produced by periodically varying the conditions under which the X-ray images are produced. Such conditions can be varied by moving different X-ray filters into and out of the path of the X-rays. The sequential first and second X-ray images are converted by a television camera into first and second video image signals, which are supplied sequentially to a video subtraction detector preferably utilizing a first video storage tube in which the video images can be stored in the form of electrical charges on a target. Such target preferably has a dielectric layer over a conductive backplate.

After the first video signal has been written on the target, the second video image is written with the result that the storage tube develops a first differential video signal representing the difference between the first and second video image signals. The first differential video signal is then written positively on an integrating and subtracting storage device, preferably utilizing a second storage tube having a target which includes a mosaic of dielectric islands on a conductive backplate. The first storage tube then develops a second differential video signal representing the difference between the second and first video image signals. Such second differential video signal is written subtractively on the target of the second storage tube.

This series of operations has the effect of integrating the differential portions of the first and second video signals, while subtractively combining the nondifferential portions. The integrated image can be read as desired from the second storage tube and supplied to a television monitor which will produce a visible dis play of the enhanced differential image features.

Further objects, features and advantages of the present invention will appear from the following description, taken with the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of a system or apparatus for the enhancement of differential images to be described as an illustrative embodiment of the present invention.

FIG. 2 is a diagrammatic representation of the first video storage tube employed in the video difference detector of FIG. 1.

FIG. 3 is a diagrammatic representation of the second video storage tube employed in the integrating and subtraction storage devices of FIG. 1.

FIG. 4 is a graph representing the writing characteristics of the second storage tube, such graph being effective to illustrate how the images can be written either positively or negatively on the second storage tube.

FIG. 5 is a set of waveform and operational diagrams illustrating the operation of the system of FIG. 1.

FIG. 6 is a graph in which the X-ray attenuation coefficients of certain materials are plotted as a function of X-ray energy to illustrate the manner in which differential X-ray images may be produced.

FIG. 7 is a graph illustrating the manner in which X- rays of different energy distributions can be produced by using different X-ray filters.

As just indicated, FIG. 1 illustrates a system or apparatus 10 for the enhancement of differential features in periodically variable images. In this case, the images are produced by an X-ray system 12 utilizing an X-ray source 14, which may simply comprise an ordinary X-ray tube. The X-ray source 14 is adapted to be energized by a high voltage source 16, preferably in the form of an X-ray pulser adapted to supply high voltage pulses to the X-ray source 14, whenever a control pulse or signal is supplied to the X-ray pulser 16 over a control line 18.

The X-rays from the X-ray source 14 pass through the patient or subject 20 to be X-rayed and then impinge upon an intensifier screen 22 which produces a visible X-ray image.

Various means may be employed to cause the X-ray image to be periodically variable so that the X-ray image will have differential features which may be enhanced by the system 10. For example, the X-ray pulser 16 may be constructed and arranged to supply sequential pulses of different voltages to the X-ray source 14 so that the energy of the X-rays will be varied in a timedependent manner. It is also possible to switch between two or more different X-ray sources which produce X- rays of different energies. Such X-ray sources may be of the monochromatic of monoenergetic type.

Perhaps the easiest method of changing the energy of the X-rays is by variable filtration. This method is employed in the X-ray system 12 of FIG. 1, in which a variable filter device 24 is adapted to be interposed between the X-ray source 14 and the patient 20. The filter device 24 comprises at least one filter which can be moved into and out of the beam of X-rays.

As shown, the variable filter device 24 comprises a rotatable filter disc 26 having at least two filter segments 26a and 26b, which contain different materials having different characteristics as to X-ray absorption so that the energy of the X-rays passing through the filter disc will be changed when the filter segments are changed. For example, the filter segments 26a and 26b may contain cerium and iodine. Various other materials may be employed as desired. The cerium may be in the form of cerium foil or a layer of coating of cerium dioxide. The iodine may be in the form of sodium iodide applied as a coating or layer with a suitable binder on a supporting member having a low X-ray absorptron.

FIG. 7 illustrates the different X-ray energy distributions which are produced by using filters containing cerium and iodine. It will be seen that FIG. 7 comprises a curve 28 which represents the energy distribution of the unfiltered X-rays. A second curve 28a represents the energy distribution when the cerium filter is used, while a third curve 28b represents the energy distribution when the iodine filter is used. It will be noted that the curves 28a and 28b constitute relatively sharp peaks so that they represent X-ray spectra which may be regarded as quasi-monoenergetic. The peak of the cerium curve 28a is at higher energy then the peak of the iodine curve 28b.

Provision is made for moving the filter segments 26a and 26b into and out of the X-ray beam. In this case, the filter disc 26 is adapted to be rotated by a motor 30 connected to the disc 26 by a shaft 32. For synchronizing purposes, a cam 34 may be mounted on the shaft 32 to operate a control switch 36. The cam 34 has two different lobes 34a and 34b corresponding to the filter segments 26a and 26b. Various other means may be provided for synchronizing purposes such as a commutator, or optical encoder.

The absorption of the X-rays by the patient or subject varies slightly when the two filter segments 26a and 26b are being used due to the difference in the energy levels of the X-rays. While this is true to a slight extent as to ordinary soft tissue, it is true to a greater extent as to portions of the patients body which contain significant quantities of a contrast medium, such as iodine.

These differences in X-ray absorption are illustrated in FIG. 6, in which the X-ray absorption coefficient (K) is plotted against the X-ray energy for water and iodine. The absorption curve for water is approximately applicable to soft tissue. It will be noted that the absorption curve for iodine has a discontinuity 38 at a particular X-ray energy. This discontinuity 38 is often referred to as the K edge.

The use of the cerium and iodine filters 26a and 26b has the effect of shifting the X-ray energy above and below the discontinuity or K edge 38 so that there will be a significantly greater differential between the two X-ray images as to iodine containing tissue than as to ordinary soft tissue. The iodine containing tissue may be the thyroid gland, for example, which naturally contains iodine in quantities which are sufficient to render the thyroid gland clearly visible by the system of the present invention. Alternatively, iodine may be injected in small quantities into the circulatory system so that the blood vessels will be rendered visible.

Various other filters and contrast media may be employed. For example, xenon gas in small quantities may be inhaled by the patient to render the respiratory system visible.

The X-ray images on the intensification screen 22 are converted into video image signals by a television system 40, including a television camera 42 which may be fo conventional construction. The first and second X-ray images, produced by the use of the first and second filters 26a and 26b, produce first and second video signals which contain small differences corresponding to the differences between the X-ray images.

The video signals from the TV camera 42 are amplified by a video amplifier 44 and are supplied through a video switch or gate 46 to a video difference detector 48. The video switch 46 is controlled by pulse signals received over a control line 46a so that the video signals will be supplied to the video difference detector 48 on a selective basis. The video difference detector is also under the selective control of voltage pulses or signals supplied by a control line 48a.

The first and second video image signals are supplied sequentially to the video difference detector 48, which is constructed and arranged to produce an output corresponding very closely to the difference between the first and second video image signals. Thus, the differen tial features of portions of the video image signals are enhanced, while the identical or non-differential features are cancelled or nearly so.

The first differential video signal, as thus produced by the video difference detector 48, is supplied through another electronic video switch or gate 50 to an integrating subtraction and storage device 52. The video switch 50 is under the selective control of pulses received over a control line 50a. Similarly, the storage device 52 is selectively controlled by pulses or voltages received over a control ine 52a.

When the first differential video signal is supplied to the storage device 52, an electronic image corresponding to such signal is written and stored in a positive sense in the storage device 52.

It will be recalled that the first differential video signal represents the difference between the first and second video image signals which are supplied sequentially to the video difference detector 48. A second differential video signal of opposite phase is then produced by reversing the sequence so that the second video image signal is followed by the first video image signal, as applied to the video difference detector 48.

Putting this another way, the first and second video differential signals are sequentially produced by supplying the first video image signal, followed by the second video image signal, followed by the reapplication of the first video image signal.

After the first video difference signal has been written positively in the storage device 52, the second video differential signal is written negatively therein. Due to the opposite phase of the first and second signals, the differential portions of the signals are integrated by the storage device 52, while the small remaining nondifferential portions are subtractively combined. Thus, a further cancellation of the non-differential portions is produced by the storage device 52. In this way, the storage device 52 still further enhances the contrast between the differential and non-differential portions of 5 the video signals.

The storage device 52 is read to produce an output video signal representing the integrated and subtracted image stored in the storage device. This output video signal is supplied to a television monitor 54 which produces a visible display of the image stored in the storage device 52. In such image, the differential features of the periodically variable X-ray images are greatly en'- hanced so that they become clearly visible, even though they may have been scarcely visible or even invisible in the original X-ray images.

The control pulses or signals for controlling the oper- 76a and 78a. Further details of the control pulse generator 66 will be given presently.

FIG. 2 illustrates an electronic storage tube 80 which may be employed in the video difference detector 48 of FIG. 1. It will be understood that other difference detecting devices may be employed.

The storage tube 80 is of a type which has been used as a moving target indocator for radar systems or other surveillance systems. Tubes of this type are manufactured by Princeton Electronic Products, Inc. and Hughes Aircraft Company.

As shown, the storage tube 80 is employed in an operating circuit including a video input line 82 and a video output line 84. The video signals from the TV ating sequence of the system 10 may be supplied by a control pulse generator 56. The previously mentioned control lines 46a and 50a are illustrated as extending from the control pulse generator 56 to the electronic video switches or gates 46 and 50.

The vertical synchronizing pulses for the television system 40 are supplied to the control pulse generator 56 for synchronizing purposes over a control line 58 extending from the vertical sync pulse generator 60. The sync pulses are also supplied to the sweep generator 62 of the TV system. Such sweep generator 62 supplies the sweep or scanning signals to the TV camera 42, the video difference detector 48 and the integrating subtraction and storage device 52.

The control pulse generator 56 is also synchronized with the rotation of the X-ray filter disc 26. For this purpose, a control line 64 extends between the camoperated switch 36 and the control pulse generator 56.

In the illustrated system 10, the video difference. detector 48 and the integrating subtraction and storage device 52 are controlled by supplying different operating voltages thereto. For this purpose, the system 10 may include a power supply 66. In this case, the power supply 66 has outputs to supply 6, 20, 30, 50 and 340 volts. It will be understood that these voltages may be varied and that the above-mentioned voltages are cited merely by way of example.

Means are preferably provided to switch or key the operating voltages so that they may be selectively supplied to the control lines 48a and 52a leading to the video difference detector 48 and the integrating subtraction and storage device 52. In the illustrated system 10, such means take the form of electronic switches or gates 68, 70, 72, 74, 76 and 78. As shown, the electronic switches 68, 70, 72, 74 and 76 are connected to the 6, 20, 30, 50 and 340 volt outputs of the power supply 66, respectively. The electronic switch 78 is also connected to the 340 volt output. The control line 48a for the video difference detector 48 is connected to the electronic switches 72 and 76, which thus control the supply of 30 volts and 340 volts to the detector 48. The control line 52a for the integrating subtraction and storage device 52 is connected to the electronic switches 68, 70, 74 and 78, which thus control the supply of 6, 20, 50 and 340 volts to the storage device 52.

The electronic switches 68, 70, 72, 74, 76 and 78 are preferably controlled by pulses or signals supplied by the control pulse generator 56. Such control pulses may be supplied over control lines 68a, 70a, 72a, 74a,

camera 42, arriving through the amplifier 44 and the video switch 46, are applied to the input line 82. During the first TV frame, similar video signals appear on the ouput line. However, the magnitude of the output video signals decreases with each passing frame, as the tube 80 comes to equilibrium. Any changes in the input video signals are transmitted with full magnitude, but the unchanged or non-differential portions of the video signals are largely cancelled out.

The first storage tube 80, as illustrated in FIG. 2, has a spectial target 86 but otherwise may be similar in construction to a conventional vidicon cathode ray camera tube used in TV cameras. The target 86 is scanned by an electron beam or cathode ray, produced by a conventional electron gun 88, which may include a cathode 88K and three grids 88G1, 88G2 and 8863. The input line 82 is preferably connected to the cathode 88K, and also preferably to the first and second grids 88G1 and 8862. Thus, the electron beam is modulated by the video input signals.

Means are provided to deflect the electron beam in the storage tube 80. Either magnetic or electrostatic deflection may be employed. For illustrative purposes, the storage tube 80 is shown as having horizontal and vertical deflection plates 90 and 92, which may be supplied with sweep signals from the TV sweep generator 62 of FIG. 1. However, magnetic deflection coils can be employed instead of the deflection plates. One or more magnetic focusing coils may also be employed.

The target 86 of the first storage tube 80 may take the form of an electrically conductive backplate or signal plate 94 having a thin dielectric layer or facing 96 thereon. A collector electrode 98 is provided adjacent the target 86.

The conductive backplate 94 may be made of doped silicon, while the dielectric facing 96 may comprise a thin layer of silicon dioxide (SiO grown thereon. The dielectric layer 96 is adapted to be charged electrostatically by the electron beam so that electrostatic television images can be written electrostatically on the layer 96 by the electron beam.

In the illustrative arrangement of FIG. 2, a load in the form of a resistor 100 is connected between the backplate 94 and the control line 48a, to which different power supply voltages may be applied by the electronic switches 72 and 76. If desired, a coupling capacitor 102 may be connected between the backplate 94 and the video output line 84.

There is capacitive coupling only between the charged front surface of the dielectric layer 96 and the backplate 94.

The first X-ray image, produced, for example, with the cerium filter 26a, is converted into video signals by the TV camera 42 and may be written in the form of electrostatic charges on the dielectric layer 96 of the target 86 in the storage tube 80. During the first television frame, the electron beam distributes charges on the dielectric layer 96, corresponding to the video signals. Due to the capacitive coupling through the thin dielectric layer 96, the charging of the layer 96 produces displacement currents to the backplate 94 through the load resistor 100 so that video signals are supplied to the output line 84. During subsequent frames, a state of equilibrium tends to be established between the video voltages on the cathode 88K and the voltages due to the charges on the front surface of the dielectric layer 96. As the state of equilibrium develops, the charging currents along the electron beam tend to drop to zero so that the video output currents also tend to drop to zero. Thus, the stable or nondifferential portions of the video signals tend to be cancelled out.

Before full equilibrium is established, the first X-ray image is replaced with the second X-ray image by switching from the cerium filter 26a to the iodine filter 2612. Due to the resulting change in the energy of the X-rays, there is generally some change in the X-ray image so that the second image differs slightly from the first image, particularly in areas where a contrast agent, such as iodine, may be present.

The changes in the video signals on the cathode 88K of the first storage tube 80 produce changes in the charges on the front surface of the dielectric layer 96. The changes in the charges produce corresponding video output signals on the blackplate 94 and the output line 84.

Thus, the first storage tube 80 produces differential video output signals corresponding to the differential features, as between the first and second X-ray images. The non-differential features are largely cancelled out. Because full equilibrium was not achieved in the writing of the first video image, the cancellation is not complete. However, it is desirable to avoid reaching full equilibrium, because with only partial equilibrium it is possible to detect video signal changes which are both positive and negative in sign.

The first storage tube 80 produces a great enhancement of the differential portions of the video signals, corresponding to the differential features of the X-ray images. The differential video signals from the first storage tube 80 are integrated by the integrating subtraction and storage device 52.

The storage device 52 may comprise a second electronic storage tube 104, as shown in FIG. 3. While various cathode ray storage tubes may be employed, the illustrated tube is of the silicon storage type having a special target 106 comprising an electrically conductive backplate or signal plate 108 with a moasic 110 thereon of dielectric islands. Preferably, the backplate is made of doped silicon, while the mosaic 110 comprises islands of silicon dioxide (SiO selectively grown thereon.

Aside from the target 106, the second storage tube 104 may be similar to a conventional vidicon cathode ray camera tube as used in television cameras. A collector electrode 112 is provided adjacent the target 106.

The mosaic on the target 106 is scanned by an electron beam or cathode ray produced by a conventional electron gun 114 having a cathode 114K and three 8 grids ll4G1, ll4G2 and 114GB. Either magnetic or electrostatic deflection may be employed. In this case, deflection coils 116 are provided to produce magnetic deflection. Alignment coils 118 may also be provided.

The differential video signals from the first storage tube may be supplied to the second storage tube 104 by way of the output line 84, the video switch 50, and an input line 120, which in this case is connected to the first grid 114G1 of the second storage tube 104. Thus, the electron beam current is modulated by the differential video signals.

The output of the storage tube is preferably derived from the backplate 108 of the target 106. Thus, the backplate 108 is coupled to an output line 122, preferably through a coupling capacitor 124. In the illustrated arrangement, a load in the form of a resistor 126 is connected between the backplate 108 and the control lead 52a of FIG. 1. It will be recalled that the various power supply voltages are supplied to the lead 52a by the electronic switches 68, 70, 74 and 78.

The second storage tube 104 is employed to write electrostatic images on the mosaic of the target 106 corresponding to the differential video signals from the first storage tube 80 by applying such differential video signals to the input line 120, which transmits the signals to the first grid 1l4G1 of the storage tube 104. The images can be written in either a positive or a negative sense, depending upon the voltage which is supplied to the backplate 108.

The ability to write either positively or negatively is shown by the characteristic curve of FIG. 4, in which the secondary emission coefficient of the target island mosaic 110 is plotted as a function of the target island mosaic voltage. When the electron beam impinges upon the target islands of the mosaic 110, secondary electrons are emitted by the target islands in increasing numbers with increasing target island voltage.

As plotted in FIG. 4, the secondary emission coefficient in the net number of secondary electrons emitted for each primary electron supplied by the electron beam. When the coefficient is greater than zero, the electron beam writes images with positive charges on the target mosaic 110 because each primary electron from the electron beam causes the emission of more than one secondary electron from the target island mosaic 110. When the coefficient is negative, the electron beam writes images with negative charges because each primary electron causes the emission of less than one secondary electron on the average. The backplate voltage at which the coefficient is zero may be called the crossover voltage. Above crossover, which is about 30 volts for the characteristic curve shown in FIG. 4, the electron beam causes a net deposit of positive charges on the islands of the mosaic 110. Below crossover, the electron beam causes a net deposit of negative charges.

While the differential video signals produced by the first storage tube 80 can be written on the target 106 of the second storage tube 104 in either positive or negative charges, it is preferred to write the differential video signals in positive charges. This is done by applying the differential video signals to the input line 120, while maintaining the target voltage above the crossover voltage. For example, a target voltage of 50 volts is employed in this case and is supplied to the backplate 108 by the electronic switch 74. The control pulse generator 56 supplies a pulse over the control line 74a to the electronic switch 74 to actuate it so that the 50 volt output of the power supply 66 will be connected to the line 52a leading to the backplate 108 of the second storage tube 104.

The electrostatic image stored on the target mosaic 110 of the second storage tube 104 can be read by applying a low voltage to the backplate 108, while scanning the target mosaic 110 with the electron beam. This reading procedure produces video signals on the backplate 108 because the electrical charges on the target mosaic 110 modulate the electron beam as it is caused to scan the target mosaic. In this case, for example, a backplate voltage of 6 volts is employed as the reading voltage. It will be understood that this voltage may be varied over a considerable range. When it is desired to read the image stored on the target mosaic 110 on the second storage tube 104, the 6 volt output of the power supply 66 is connected to the control line 52a by the electronic switch 68 in response to a control pulse supplied over the line 68a by the control pulse generator.

During the reading operation, the video signals from the backplate 108 of the second storage tube 104 are supplied to the television monitor 54 over the output line 122. The visible image produced by the monitor 54 corresponds to the difference between the first and second X-ray images produced by the use of the different filter elements 26a and 26b.

It will be recalled that the difference between the first and second video signals, corresponding to the first and second X-ray images, is produced by supplying the first and second video signals sequentially to the first storage tube 80. After this difference has been obtained, it is preferred to reapply the first video signals to the input line 82 of the first storage tube 80 so as to obtain the inverse difference. Thus, the sequential application of the second and first video signals to the first storage tube 80 results in the production of second differential video output signals on the output line 84 of the first storage tube 80. The phase of the second differential video signals is opposite from the phase of the first differential video signals. Thus, a positive going differential feature in the first differential video signals will be replaced by a negative going differential feature in the second differential video signals, and vice versa.

Inasmuch as the first differential video signals were written in a positive sense on the target mosaic 110 of the second storage tube 104, it is preferred to write the second differential video signals in a negative sense, by reducing the backplate voltage to a value below the crossover voltage, while modulating the electron beam with the second differential video signals. In this case, for example, a power supply voltage of volts is supplied to the backplate 108 by the electronic switch 70 over the supply line 52a in response to a control pulse supplied by the control pulse generator 56 over a control line 70a. Due to the low backplate voltage, the electron beam of the second storage tube 104 writes the second differential video signals as negative charges, which tend to neutralize or cancel the previously written positive charges. Thus, the nondifferential features of the second video signals tend to cancel out the corresponding non-differential features of the first video signals because these non-differential features are approximately equal and opposite in sign.

However, the differential features in the second differential video signals, being opposite in phase from the corresponding differential features of the first differential video signals, do not have the same tendency to neutralize or cancel the previously written signals. Instead, the differential features, as stored on the target mosaic 110, are enhanced in contrast and visibility relative to the background of non-differential features.

This cycle may be repeated as many times as desired to produce progressive integration of the differential features as stored on the mosaic 110 of the second storage tube 104. With each cycle, the contrast between the differential features and the non-differential features is enhanced. Generally, the full contrast provided by the television system is achieved after 2 to 50 cycles, depending on the initial contrast of the differential feature between the first and second X-ray images. With the system of the present invention, an X-ray image differential amounting to loss than 1% can be integrated and enhanced to full contrast.

It will be understood that in each cycle the first and second video signals, corresponding to the first and second X-ray images are supplied to the input line 82 of the first storage tube 80, which then produces first differential video signals at the output line 84. These differential video signals are then written as positive charges distributed on the target mosaic 110 of the second storage tube 104 with the backplate voltage above crossover. The first video signals are then reapplied to the input line 82 of the first storage tube 80, which produces the second differential video signals on the output line 84. The second differential video signals are written as negative charges on the target mosaic 110 of the storage tube 104 with the backplate voltage below crossover. By this cycle, the differential features of the X-ray images areenhanced, while the stable or nondifferential features are largely cancelled out.

When it is desired to detect very small contrast changes in the X-ray images, the signal-to-noise ratio of the input video signals becomes a limiting factor. With a repetitive periodic contrast change, the difference detection process can be repeated several times and the resulting difference signals can be integrated on the second storage tube 104 so as to improve the signal-tonoise ratio.

In detecting very small contrast changes, it has been found that an account must be taken of the fact that the thermionically emitted cathode electrons in the first storage tube have an inherent energy distribution following the Boltzman statistical principles. Due to this factor, it is undesirable to allow the first storage tube 80 to reach a state of equilibrium between the dielectric target layer and the cathode because any small new voltage change would be buried under the Boltzman distribution and would thus be invisible. Therefore, it is advantageous to introduce the second video signals corresponding to the second image before-equilibrium is reached. Due to the fact that equilibrium is not reached, there is a residual signal in addition to the difference signal. In the second storage tube 104, the difference signal is integrated, but the residual or direct current signal is subtracted or cancelled.

In this regard, the detailed procedure or mode of operation of the system is illustrated in FIG. 5. As illustrated, the mode of operation involves a four-step cycle. FIG. 5 illustrates the first four-step cycle and part of the second cycle. It is generally desirable to use from 2 to about 50 cycles, depending upon the specific application.

As illustrated in the diagrams of FIG. 5, Images number l and number 2 are identical with the exception of an additional white bar in Image number 2. During Step I, which may comprise one television frame, the first storage tube 80 of the video difference detector is cleared by applying a high voltage, such as 340 volts, to the backplate 94. This voltage is applied by the electronic switch 76 in response to a pulse from the control pulse generator 56. The application of the high voltage causes the electron beam to cover the dielectric layer with a uniform charge.

During the first and second steps of only the first cycle, the second storage tube 104 is primed and erased as will be evident from Parts IIA and IIB of FIG. 5. The priming step involves applying a high voltage, such as 340 volts, to the backplate 108. The electron beam then distributes positive charges over the entire mosaic 110. The high voltage is applied to the backplate 108 by the electronic switch 78 in response to a pulse from the control pulse generator 56. The erasing step involves the application of a voltage substantially below crossover to the backplate 108. The electron beam then progressively removes the positive charges and distributes negative charges. In this case, the low voltage is derived from the 20 volt output of the power supply and is supplied to the backplate 108 by the elec tronic switch 70 in response to a control pulse from the control pulse generator 56.

In the second step, Image number 1, serving as a reference image, is written on the video difference tube 80, as indicated in Parts IB, IC and ID of FIG. 5. For this operation, the high voltage of 340 volts is reduced to a low voltage, such as 30 volts, on the backplate 94. This low voltage is applied by the electronic switch 72, in response to a pulse from the control pulse generator 56. The video switch 46 is actuated by a pulse from the control pulse generator so that the video signals are supplied to the cathode and first grid of the difference detector tube 80.

Part IE of FIG. illustrates the output video waveform at the backplate of the difference tube 80 as Image number 1 is written. It will be seen that the magnitude of the video signals decreases exponentially as a state of equilibrium is approached. The video output signal does not go to zero because full equilibrium is not achieved.

During the third step, involving television Frame 7, the second video signals, corresponding to X-ray Image number 2, are supplied to the cathode and the first and second grides of the video difference detector tube 80, as will be evident from Parts IC, ID and IE of FIG. 5. As a result, a large video differential output corresponding to the white bar and some residual signal from Image number 1 is produced at the output of the difference tube 80. This video differential signal is shown in Part IE, Frame 7.

As will be evident from Part IIC of FIG. 5, this differential video signal is supplied to the grid of the second storage tube 104 and is written on the mosaic 110 of the tube in positive charges. The video switch 50 is actuated by a pulse from the control pulse generator. The positive writing is achieved by applying a voltage above crossover to the backplate 108. As illustrated, such voltage is derived from the 50 volt output of the power supply 66 and is supplied to the backplate 108 by the electronic switch 74 in response to a control pulse from the generator 56.

In Step 4, the video input signals corresponding to Image number 1 are reapplied to the video difference detector tube 80, as indicated in Parts IC and ID of FIG. 5. As a result, the output video waveform shown in Part IE contains a second difference signal which is phased oppositely from the first difference signal. This difference signal is applied to the grid of the second storage tube 104, as illustrated in Part IIC, Frame 9, and is written negatively by applying a voltage below crossover to the backplate 108. Such voltage may be derived from the 20 volt output of the power supply 66 and may be supplied to the backplate 108 by the electronic switch 70 in response to a control pulse from the generator 56. The video switch 50 is actuated by the control pulse generator 56.

It will be understood that by this procedure the differential portions of the video signal are integrated and enhanced on the screen of the second storage tube 104, while the non-differential or residual portions are largely cancelled out. This will be evident from Part IID of FIG. 5, Frames 10-12.

When the second storage tube 104 is not being primed, erased or written, the second storage tube is switched to a reading mode by applying a low voltage, such as 6 volts, to the backplate 108, as will be evident from Parts [IA and IIB. This reading voltage is applied by the electronic switch 68 in response to control pulses from the generator 56. The stored video signals corresponding to Image number 1 are read in Frame 8 of Part IID. The enhanced and subtracted video signals are read in Frames 10, 11 and 12 of Part IID.

Reverting to Frame 9, Part IE of FIG. 5, it will be seen that the residual image is still in the output of the video difference detector tube 80, but, in addition, there is a small dip at the location of the white bar. This is due to the fact that the white bar introduced in Frame 7 tended to drive the dielectric of the difference detector more negative than the surrounding elements. This is beneficial because a black bar written negatively on the silicon storage tube is equivalent to subtracting less charge from the target on the negative write step and thus makes the final white bar more prominent with respect to the surrounding regions.

In the second and subsequent cycles, all four steps (with the exception of the priming and erasing of the silicon storage tube 104) are then repeated. Due to the integrating effect of the tube 104, an increasing difference signal is stored on the mosaic of the tube.

We claim:

1. Differential imaging apparatus,

comprising image producing means for alternately producing first and second sequential image with identical portions and differential portions,

means including a television system for converting said first and second images into first and second video signals,

a video difference detector,

means for sequentially supplying said first and second video signals corresponding to the first and second images to said video difference detector,

said detector being operative to produce a first differential video signal corresponding to the difference between said first and second video signals,

an image storage device,

first writing means for writing an image in a positive sense on said image storage device corresponding to said first differential video signal,

means for deriving a second differential video signal from said video difference detector corresponding to the difference between said second and first video signals,

second writing means for writing a second image in a negative sense on said image storage device corresponding to said second differential video signal whereby said image storage device additively integrates the differential portions of said differential video signals while subtractively combining the non-differential portions thereof,

reading means for reading the integrated image stored in said image storage device,

and display means for displaying said integrated image.

2. Apparatus according to claim 1,

in which said image producing means includes X-ray apparatus for alternately producing first and second X-ray images under significantly different conditions whereby said images will have differential portions.

3. Apparatus according to claim 1,

in which said image producing means comprises X-ray apparatus for alternately producing first and second X-ray images with X-rays having significantly different energies whereby said images will have differential portions.

4. Apparatus according to claim 1,

in which said image-producing means comprises X-ray apparatus for alternately producing first and second X-ray images using X-rays of different energies,

said X-ray apparatus including an X-ray source and differential X-ray filtration means for alternately filtering the X-rays from the source for'producing the different X-ray energies.

5. Apparatus according to claim 1,

in which said image-producing means comprises X-ray apparatus for alternately producing X-ray images using X-rays having different energies,

said X-ray apparatus including an X-ray source and means for alternately energizing said X-ray source with two different voltages to produce the different energies.

6. Apparatus according to claim 1,

, including output means coupled to the backplate of said cathode ray tube.

10. Apparatus according to claim 1,

in which said image storage device comprises a storage tube having a target for storing electrical charge images.

11. Apparatus according to claim 10,

in which said storage tube comprises a target having a mosaic of dielectric islands over a conductive backplate,

and means for causing an electron beam to scan said target.

12. Apparatus according to claim 11,

in which said first writing means includes means for energizing said backplate with a first voltage which is sufficiently high to produce writing with positive charges on said target by the electron beam,

said second writing means including means for energizing said backplate with a lower voltage such that the electron beam produces writing with negative charges on said target.

13. Apparatus according to claim 12,

in which said reading means includes means for energizing said backplate with a low voltage while deriving a video signal output from said backplate.

14. Apparatus according to claim 12,

including means for alternately actuating said first and second writing means through a plurality of cycles to produce enhanced integrations of the differential signal portions.

29??? UNITED STATES PATENT OFFICE (IEBTIFICATE 0F CURRECTEQN 6 Patent No. Dated y 1975 Inventor(s) tta et al It is certified that error appears in the above-identified patent g and that said Letters Patent are hereby corrected as shown below:

Column 3, line 2, "of" should be "or".

9 Column 3, line 35, insert "a" before "higher".

Column 4,. line 15, "f0" should be t "of Column 4, line 34, "of" should be "or Column. 8, line 39, "in" should be "is".

Column 10, line 15, "loss" should be "less".

Column 12, line 50, "image" should be "images".

a I I Signed and Scaled this thirteenth Day of April 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Altcsring ()jfiver (tmzmissimwr uj'lan'l'zls and Trademarks O 

1. Differential imaging apparatus, comprising image producing means for alternately producing first and second sequential image with identical portions and differential portions, means including a television system for converting said first and second images into first and second video signals, a video difference detector, means for sequentially supplying said first and second video signals corresponding to the first and second images to said video difference detector, said detector being operative to produce a first differential video signal corresponding to the difference between said first and second video signals, an image storage device, first writing means for writing an image in a positive sense on said image storage device corresponding to said first differential video signal, means for deriving a second differential video signal from said video difference detector corresponding to the difference between said second and first video signals, second writing means for writing a second image in a negative sense on said image storage device corresponding to said second differential video signal whereby said image storage device additively integrates the differential portions of said differential video signals while subtractively combining the non-differential portions thereof, reading means for reading the integrated image stored in said image storage device, and display means for displaying said integrated image.
 2. Apparatus according to claim 1, in which said image producing means includes X-ray apparatus for aLternately producing first and second X-ray images under significantly different conditions whereby said images will have differential portions.
 3. Apparatus according to claim 1, in which said image producing means comprises X-ray apparatus for alternately producing first and second X-ray images with X-rays having significantly different energies whereby said images will have differential portions.
 4. Apparatus according to claim 1, in which said image-producing means comprises X-ray apparatus for alternately producing first and second X-ray images using X-rays of different energies, said X-ray apparatus including an X-ray source and differential X-ray filtration means for alternately filtering the X-rays from the source for producing the different X-ray energies.
 5. Apparatus according to claim 1, in which said image-producing means comprises X-ray apparatus for alternately producing X-ray images using X-rays having different energies, said X-ray apparatus including an X-ray source and means for alternately energizing said X-ray source with two different voltages to produce the different energies.
 6. Apparatus according to claim 1, in which said video difference detector includes a video subtraction cathode ray tube.
 7. Apparatus according to claim 6, in which said cathode ray tube includes a target having a dielectric layer over a conductive backplate.
 8. Apparatus according to claim 7, in which said cathode ray tube comprises means for causing an electron beam to scan said dielectric layer to produce an electrical charge image on said dielectric layer.
 9. Apparatus according to claim 8, including output means coupled to the backplate of said cathode ray tube.
 10. Apparatus according to claim 1, in which said image storage device comprises a storage tube having a target for storing electrical charge images.
 11. Apparatus according to claim 10, in which said storage tube comprises a target having a mosaic of dielectric islands over a conductive backplate, and means for causing an electron beam to scan said target.
 12. Apparatus according to claim 11, in which said first writing means includes means for energizing said backplate with a first voltage which is sufficiently high to produce writing with positive charges on said target by the electron beam, said second writing means including means for energizing said backplate with a lower voltage such that the electron beam produces writing with negative charges on said target.
 13. Apparatus according to claim 12, in which said reading means includes means for energizing said backplate with a low voltage while deriving a video signal output from said backplate.
 14. Apparatus according to claim 12, including means for alternately actuating said first and second writing means through a plurality of cycles to produce enhanced integrations of the differential signal portions. 